The present invention relates to method and apparatus for measuring the lung air capacity.
For a variety of diagnostic and related reasons, it is important to be able to accurately determine the volume of air in the lungs. Such measurements are crucial in evaluating lung damage as a result of disease or trauma. The measurements are also importance in analyzing the extent to which blood is accommodated in the lungs during breathing, for example under stress conditions.
Air is typically found in the lungs in two places. First, in normal lungs, virtually all the air is found in aerated portions of the lungs, i.e., in portions of the lungs from which the air can readily be expelled and replaced. A certain amount of trapped air is often found in pathological portions of the lungs, for example, in bullae, or blisters.
The air found in the aerated portions of the lungs is termed the Functional Residual Capacity (FRC) of the lungs. The total air in the lungs, including trapped air in pathological tissue, is termed Total Gas Volume (TGV).
Various techniques have been proposed for measuring lung air volume. At least two of these techniques are in common use. The gas dilution technique, described in more detail below, makes use of a spirometer which contains a certain known concentration of a gas not normally found in the lungs, such as helium. After steady state is achieved the gas is analyzed chemically and the determined concentration of the helium is used to calculate the patient's FRC.
The plethysmography technique makes use of use of a body box. The patient is placed in a body box which is hermetically sealed. The patient then breathes through a breathing tube which is then blocked and the measured pressure in the lung and breathing fluctuations of the volume of the body box are used to determine the patient's TGV.
Each of these techniques suffers from a number of disadvantages. Thus, the gas dilution technique requires the use of certain expensive and difficult to handle gases, such as helium and xenon. Furthermore, the techniques requires the use of a gas analyzer. Finally, it is not normally possible to use the technique to measure lung capacity under stress since the measurement typically takes from 3 to 7 minutes which is ordinarily longer than the time of the stress.
The biggest disadvantage of the plethysmography technique is that it requires a large and expensive body box. Here, too it is not possible to carry out the measurement under stress conditions since the body box is confining and since stress would lead to a warming of the air in the body box, thereby reducing the accuracy of the measurements. Finally, the plethysmography technique calls on the patient to simulate normal breathing but with a blocked breathing tube which is difficult for some people to accomplish, further degrading the accuracy of the technique.
There is thus a widely recognized need for, and it would be highly advantageous to have, a technique for measuring lung capacity (TGV) which is rapid and accurate, which does not require the use of bulky and expensive equipment and which can be used under various conditions, including stress, and the like.